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Dirac materials are characterized by exceptional mobility, orders of magnitude higher than any semiconductor, due to the massless pseudorelativistic nature of the Dirac fermions. These systems being semimetallic, the lack of a genuine band-gap poses a serious limitation to their possible applications in electronics. We recently demonstrated that thin TiO2 nanowires can exhibit 1D Dirac states similar to metallic carbon nanotubes, with the crucial difference that these states lie inside the conduction band in proximity of a wide band gap. We analyze the robustness of the Dirac states respect to an Anderson disorder model and substitutional impurity and compare to different one dimensional systems. The results suggest that thin anatase TiO2 nanowires can be a promising candidate material for switching devices.
Large area transmission and section topographs of semi-insulating gallium arsenide wafers grown by the gradient freeze technique are made with synchrotron radiation at HASYLAB in Hamburg and at ESRF in Grenoble. Several high-resolution images including stereo pairs are obtained on the same film at a time. A typical dislocation line is an arc of a circle which starts from one surface and ends at the same surface. From the disappearance of the dislocation image and using the g · b = 0 criterion it is concluded that the Burgers vector b of the most common dislocations is parallel to 〈110〉. Rather large volumes of the wafer are dislocation-free. Section topographs of epitaxial wafers show defects and strain fields at the interface between an n-type substrate and the epitaxial layers grown by chemical vapor deposition. The results are compared with those obtained from detector performance measurements.
One of the main electronic noise sources of a room temperature spectroscopy system is the leakage current of a detector. It can be reduced with a pn-junction type detector structure such as a M-i-n configuration, and with cooling. In this work eight CdZnTe detectors with a M-i-n structure were fabricated by indium diffusion. The junction was characterized by a currentvoltage technique. Detector electrical, charge collection and spectroscopic properties were compared to the ones received with the traditional electroless Au contacts, before the junction formation. As a result of the indium diffusion an improved detector leakage current performance was achieved. However, a corresponding improvement in the detector energy resolution was not always observed due to the CdZnTe charge collection properties and process variables.
The electrical and charge collection properties of a semiconductor detector play an important role in a spectrometer's final performance. However, the studies of these properties often concentrate on only a few samples. In this work over 100 CdZnTe detectors from 12 different growth boules were characterized with one of the following test methods. The composition uniformity was evaluated with low temperature photoluminescence (PL) measurements. From the current-voltage characteristics the differences in CdZnTe detector resistivities were investigated. Charge collection properties, μτ-products, and energy resolutions were characterized with spectroscopic methods using an alpha and isotopic sources. A wide selection of test results are presented indicating the variety of CdZnTe material.
The use of some specific compound semiconductors in the fabrication of high energy X-ray detectors shows significant potential for X-ray astrophysics space missions. We are currently investigating three high purity crystals - CdZnTe, GaAs and TlBr - as the basis for future hard X-ray detectors (above 10 keV). In this paper we present the first results on CdZnTe and GaAs based detectors and evaluate the factors currently still constraining the performance.
Energy resolutions (FWHM) of 0.9 keV and 1.1 keV at 14 keV and 60 keV, respectively, have been obtained with an epitaxial GaAs detector, while 0.7 keV and 1.5 keV FWHM were measured at the same energies with a CdZnTe detector. Based on these results it is clear, that the next generation of X-ray astrophysics missions now in the planning phase may well consider extending the photon energy range up to ∼100 keV by use of efficient detectors with reasonable spectroscopic capabilities.
Human bocavirus (HBoV) was recently identified in respiratory samples from patients with acute respiratory infections and has been reported in different regions of the world. To the best of our knowledge, HBoV has never been reported in respiratory infections in Brazil. Nasopharyngeal aspirates were collected from patients aged <5 years hospitalized in 2005 with respiratory infections in Ribeirão Preto, southeast Brazil, and tested by polymerase chain reaction (PCR) for HBoV. HBoV-positive samples were further tested by PCR for human respiratory syncytial virus, human metapneumovirus, human coronaviruses 229E and OC43, human influenza viruses A and B, human parainfluenza viruses 1, 2 and 3, human rhinovirus and human adenovirus. HBoV was detected in 26/248 (10·5%) children of which 21 (81%) also tested positive for other respiratory viruses. Despite the high rates of co-infections, no significant differences were found between HBoV-positive patients with and without co-infections with regard to symptoms.
Five natural compounds possessing antiparasitic activity were isolated from East African plants and tested, together with some semisynthetic derivatives, against micro-organisms and arthropods of agricultural interest. Anonaine (1), from Annona squamosa, canthin-6-one(8) and 5-methoxycanthin-6-one (9), from Fagaropsis angolensis, showed fungicidal activity. E,Z-matricarianol (4), from Hoehnelia vernonioides was fungicidal, Insecticidal and acarlcldal. Caryophyllene oxide (12), isolated from Uvaria sp., turned out to be insecticidal and antifeedant. Canthin-6-one (8) and its dihydroderivative 11 were the most effective antimicrobial compounds tested. The possibility ofusing some of the isolated compounds as leads for new synthetic chemicals is discussed.
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